Buffer Dewar

A 500 l buffer dewar is used to store LHe for the target magnet and refrigerator. The liquid helium in the buffer dewar is constantly replenished by a PID loop operating the supply JT valve EV9180. The liquid level inside the dewar is measured by LL91101. A second PID loop sets the pressure inside the dewar by throttling the flow of cold vapor back to ESR via EV9178. The position of both valves, in percent open are determined by LVDTs read back to the control computer via ADC channels. Silicon diode thermometer TD9180 (TD9178) monitors the temperature of the helium supplied to (returned from) the dewar. The thermometers are read by an Oxford Instruments ITC-502 temperature controller. The dewar pressure is measured by a 0-2000 mbar pressure transducer, PI91105, mounted on the cryostat gas panel. The option of using room temperature helium gas to pressurize the buffer dewar exists via solenoid valve SV91108, also mounted on the gas panel.

A LHe transfer line is used to provide LHe from the 500 l buffer dewar to the magnet reservoir. A stepper-motor driven needle valve, EV91102 is mounted at the buffer dewar end of the line to control the flow of LHe to the magnet reservoir (see Section 8). An optical encoder is mounted to the valve stem to measure its exact position (in # of turns open). The encoder is read by a US Digital ED2 display, connected to the IOC over a serial interface.

Because the buffer is directly connected to the Hall C cryogenic distribution system, improper operation of its control might produce a serious disruption of cryogenic flow to the hall (or even all three halls). Please refer to Section 10 at the end of this manual for further information.